The present invention relates to magnetic resonance imaging (MRI) and functional magnetic resonance imaging (fMRI) and in particular to a pulse sequence and methodology for acquiring image data contemporaneously with information about variations in magnetic field homogeneity.
Magnetic resonance imaging is used to generate medical diagnostic images by measuring faint radio frequency (RF) signals (magnetic resonance) emitted by atomic nuclei in tissue (for example, water protons) after radio frequency stimulation of the tissue in the presence of a strong magnetic field.
The location of the precessing protons is made possible by the application of orthogonal magnetic gradient fields which serve to “encode” the spins according to frequency, phase, and/or slice. The combination of the radio frequency stimulation and the applied gradient fields is termed a pulse sequence.
The acquired signal from the spins (termed a nuclear magnetic resonance (NMR) signal) provides data in “k-space”, a mathematical construction in the frequency domain. A two-dimensional Fourier transform of the k-space data produces the actual image. It will be understood, therefore, that the k-space data does not represent the image itself, but represents the spectral components of the image with the center of k-space representing low frequency spatial components of the image, and the outer portions of k-space representing the high frequency spatial components of the image.
The impressing of spatial location information onto the spins of the NMR signal by the applied magnetic gradients makes it extremely important that all applied magnetic fields (including the polarizing magnetic field B0 and the gradient magnetic fields Gx, Gy, and Gz) be well characterized. For this reason, and particularly for the B0 field, it is well known to incorporate shimming coils into the design of a magnetic resonance imaging machine which serve to correct for inhomogeneities in the B0 field through the application of one or more superimposed shimming fields.
A number of techniques are known by which to measure inhomogeneities of the magnetic field and thus to calculate the currents needed for the shimming coils. For example, special pulse sequences detecting phase differences in the MRI measurements of a homogenous phantom, for example, a tank of water, may be used to deduce variations in the magnetic field of the MRI system.
Shimming of the MRI system may be accomplished with great precision, however, the magnetic homogeneity is upset almost immediately upon insertion of a human subject whose tissue distorts the field. In order to address this problem, it is known to create a magnetic field map once a subject is in position in the MRI machine to compensate for this distortion. Such compensation is particularly important for echo planar imaging (EPI) and spiral imaging where the precessing nuclei have a long period of time in which to be influenced by the magnetic field, and thus to accumulate errors caused by inhomogeneity.